Molten metal, and in particular molten aluminum, frequently contains impurities which are detrimental to the resulting cast products. These impurities may be oxides of the metal, other inclusions introduced by fluxes used during melting, or products formed by interaction with the furnace lining.
One technique for removing oxides and other impurities from molten metal is taught in U.S. Pat. No. 4,113,241 issued to Dore. This patent teaches placing a floatable crucible in a bath of molten metal. A portion of the crucible is porous and serves as a filter. The molten metal will filter through the porous filter material and thereby fill the crucible. The metal to be cast is ladled from the crucible. Since the crucible is floatable it is buoyant and the rate of fill will be limited by the rate at which the crucible settles into the molten metal bath. For the crucible of the Dore patent to float in molten aluminum it must be made of a light material, such as graphite, which may fracture if impacted by the ladle during removal of the molten metal from the crucible.